A boiler is a pressurized system in which heat is transferred to a fluid. Water is typically used as the fluid for economical reasons and its suitable thermodynamic characteristics. The water is heated by heat transferred to the water from a source of higher temperature (i.e. a flue gas), which is usually produced by the combustion of fuels. The water may be heated to a suitable temperature or heated such that it is vaporized to a steam. Accordingly, connections are made for the off-take of the heated water or the generated steam and for the replenishment of water. The heated water or generated steam can be used as a heating medium or as a working fluid in another device that converts thermal energy to mechanical work. The water also acts to cool the heat-absorbing surfaces of the boiler while the water circulates through the boiler. The heated water or generated steam, as the case may be, is removed at a controlled rate that is comparable to the rate of heat addition to the water so that the pressure in the water-vessel can be maintained at a desired, safe value.
A water-tube boiler is a particular type of boiler that includes a plurality of small-diameter, water-containing tubes arranged in a certain configuration to allow hot gases to pass over the outer surface of the tubes thereby transferring heat to the water contained inside the tubes. The water is confined in the small tubes such that the water flows upwards in a rapidly controlled manner. Water-tube boilers are typically economical for heat capacities greater than 20,000 pounds of steam per hour.
Evolution in water-tube boiler design for higher pressures and capacities has led to use of steel and increasing the length, number and orientation of the tubes. In particular, bent-tube, water-tube boilers are used for high-pressure designs. Bent-tube water-tube boilers terminate in upper and lower steam and water drums (also known as manifolds). The bent-tube water-tube boilers also have sloped tubes that reduce the possibility of steam pocket formation thereby increasing the rate of heat absorption.
Water-tube boilers can operate at an efficiency, defined by the ratio of thermal energy output to heat input, of up to 80 percent. However, to obtain a higher throughput of heated water or generated steam, either a higher efficiency boiler is needed, which, if conventionally designed, results in increased costs due to the need for additional more expensive equipment, or a larger boiler is needed which also results in increased costs due to the need for more material.